The real physics of the lemming world is embodied in the Psygnosis
program for playing the machine side of lemming games. Presumably
there is a general lemming mover, and the designers of particular
games build structures with pictures glued onto them. Let's call it
the lemming interpreter, since it also interprets the actions of
players. These actions consist almost entirely of designating
particular lemmings as having powers selected from the list of 9
possibilities.

However, the human player doesn't know the lemming interpreter, and it
wouldn't do him much good if he did. What the human knows is the
common sense physics of the lemming world. Our goal is to find an
epistemologically adequate [McCarthy and Hayes, 1969] way of expressing
this physics. Epistemological adequacy requires that the facts that
are actually observable be expressible and that the general facts
giving the effects of actions be expressed in a way that makes
the observations useful.

Here are some aspects of the common sense physics of the lemming world.

Lemmings move to the right or left and sometimes climb. I
suppose lemmings cannot stand still except for blockers and have
only one possible speed. Players use this fact in order to time
explosions. If a lemming is triggered while at place x to explode
5 seconds later at place y, triggering at x will always result
in an explosion at y.

Except for blockers, lemmings don't interact with each other,
e.g. they interpenetrate rather than collide.

The lemming world has kinematics but apparently not dynamics.
Thus the program uses velocities but not forces and accelerations.
Our own projections of plans is rarely even kinematic but merely
geometric. By non-kinematic I mean that the velocities of the
lemmings are not used in projection.

Lemmings fall at constant velocity but are destroyed if they
fall from too great a height.

Bridges are infinitely strong and don't break under the weight
of any number of lemmings.

Explosions kill only the lemming that has been told to explode.
Others are unharmed even if they are within the explosion zone.
Structures that seem to have been supported by structure that is
destroyed do not collapse but ``hang in the air''.

There seems to be no random element in lemming games except that
randomness coming from the human player. As is usual in sports,
this permits the human to learn precise procedures from repeated
plays of a game. Thus he can learn the best place to start a
bridge.

Lemming time is sometimes important. For example, a digger
needs a certain amount of lemming time before the hole is deep
enough to trap other lemmings. Before that, a walker falling into
the hole will climb on out of it. On a larger scale, the total lemming
time available for a game sometimes affects strategy.